The present invention relates to a joint prosthesis comprising a first part with a cylindrical socket and a second part with a cylindrical head, the cylindrical head and socket being complementary to one another, in such a manner that together they are able to form a linear hinge which extends in the longitudinal direction of the head/socket.
A prosthesis of this nature is known, in the form of a finger joint prosthesis, from U.S. Pat. No. 3,991,425. A prosthesis of this nature, of at least a prosthesis which appears extremely similar, is also known under the name Djoa prosthesis. The Djoa prosthesis comprises a cylindrical roll surface in which a slot is formed. A hollow cylindrical socket runs over the cylindrical roll surface, with a projection which projects out of the socket fitting into the slot. The purpose of this projection is to prevent the socket from moving sideways, i.e. in the longitudinal direction of the linear hinge, with respect to the cylindrical roll surface, in order to ensure stability of the joint prosthesis in relation to loads which act in the longitudinal direction of the hinge. However, a drawback of this design is that in the event of such loads a force is exerted on the underlying bones to which the joint is attached, via the slot with the projection situated therein. As a result, the stems may be dislodged from the underlying bone or may break off, which would lead to considerable damage to the underlying bone.
GB patent specification 1,530,301 has disclosed a finger prosthesis which differs from the joint prosthesis according to the present invention in particular by the fact that the socket parts are fixed to the head parts by means of a snap-action connection, i.e. the socket parts extend over more than 180xc2x0, as is clear from the figures. Owing to the snap-action connection, this prosthesis, in the implanted position, does not allow any movement of the prosthesis parts with respect to one another apart from the pivoting movement about the longitudinal axis of the head parts/socket parts. It is not possible for the prosthesis parts to move apart from one another and to pivot with respect to one another transversely to the hinge direction.
DE-Offenlegungsschrift 2,814,752 has disclosed a two-part joint prosthesis, in which the first part comprises a fork with a socket part inside. The second part is provided with a head part which can be fixed to the socket part by means of a snap-action connection. Since the head part is of spherical or convex design, and the socket part is shaped accordingly, this joint prosthesis has two degrees of freedom. In addition to the main hinged movement, limited pivoting with respect to the main hinged movement is also possible. The prosthesis disclosed by DE-2,814,752 cannot move apart in the implanted position.
The object of the present invention is to provide a joint prosthesis which, on the one hand, provides so-called medial/lateral stability, so that it is impossible for the moving parts to move in the longitudinal direction of the hinge with respect to one another, and, on the other hand, allows transverse forces, acting in the longitudinal direction of the hinge, on the bones which are connected by the joint without this inevitably leading to excessive load on the connection between the joint parts and the respective bones to which they are attached.
According to the invention, this object is achieved by means of a special design of the joint prosthesis, and in particular by the fact that:
the cylindrical socket extends over at most 180xc2x0 in the circumferential direction,
the cylindrical head is provided with a radial thickened section which extends in the circumferential direction, and
the cylindrical socket is provided with a radial recess which extends in the circumferential direction and in which the radial thickened section can be accommodated, preferably with the cylindrical head bearing inside the cylindrical socket.
In this design, since the cylindrical socket extends over at most 180xc2x0, the cylindrical head can be removed from the cylindrical socket by pulling it out in a direction which is parallel to or lies between the tangents at the free ends of the extent of the cylindrical socket. When the bone to which the cylindrical socket is attached extends in this direction, this essentially means that this bone is pulled away from the joint in its longitudinal direction. However, this also means that when, in this position, a transverse force which is directed in the longitudinal direction of the linear hinge acts on the said bone, this bone can undergo a pivoting movement, during which the cylindrical head comes out of the socket at one end of the linear hinge and is pressed against the socket at the other end of the linear hinge. Consequently, the action of such a transverse force on the connection between the prosthesis parts with the respective bones is reduced considerably. These transverse forces can then be absorbed entirely by the natural, or possibly artificial, ligaments which hold the joint together. If the cylindrical socket extends over less than 180xc2x0, the area in which such a tilting movement of the bone which is attached to the cylindrical socket with respect to the linear hinge can take place will be increased in size, as will be clear. However, an important advantage of the joint prosthesis according to the invention is that this so-called tilting freedom which has been outlined above decreases as the bone which is attached to the cylindrical head is situated further and further outside the area defined by the tangents at the free ends of the cylindrical head. Depending on the exact orientation of the cylindrical socket with respect to the underlying bone to which this socket is attached, a moment with respect to the longitudinal axis of the said bone will act on the said bone. Such a rotation can then be absorbed, in the case where there is a joint at the other end of the said bone, by the said joint or by the ligaments associated with the said joint. Generally, the cylindrical socket will be oriented in such a manner, with respect to the bone to which it is attached, that the cylindrical head can be removed from this socket when the two bones are situated essentially in line with one another. As will be clear to a person skilled in the art, such a joint prosthesis according to the invention can generally be used with very great success for finger joint prostheses, toe joint prostheses, and possibly elbow or knee prostheses.
In order to achieve a so-called pivoting freedom or tilting freedom of the joint prosthesis which is equal in both opposite directions (also known in the specialist medical field as the medial and lateral directions), it is advantageous, according to the invention, if the radial thickened section and the radial recess are arranged centrally, as seen in the longitudinal direction of the linear hinge.
In order to ensure that after a so-called tilting or pivoting of the joint parts with respect to one another, the cylindrical head moves correctly back into the cylindrical socket (i.e. with their cylinder axes essentially coinciding), it is advantageous, according to the invention, if the radial thickened section and/or the radial recess are formed so as to be self-aligning with respect to one another. It will be clear to a person skilled in the art that a self-aligning effect of this nature can be achieved in a wide variety of ways by suitable shaping of circumferential sides, which extend in particular in the circumferential direction, of the radial thickened section and/or the radial recess. According to the invention, the self-aligning effect can be achieved, inter alia, by providing the circumferential sides of the radial thickened section with bevels and/or rounded portions, optionally in combination with bevels and/or rounded portions on the circumferential sides of the radial recess.
In order, on the one hand, to provide sufficient medial/lateral stability, i.e. resistance to displacement of the joint-prosthesis parts with respect to one another in the longitudinal direction of the linear hinge and, on the other hand, to allow the radial thickened section and the radial recess to be self-aligning with respect to one another, it is advantageous, according to the invention, if the circumferential sides, which extend in the circumferential direction, of the radial thickened section and/or of the radial recess comprise an active circumferential side face which includes an angle of approximately 1xc2x0 to approximately 10xc2x0 with a radially oriented transverse plane. These side faces therefore do not extend at right angles to the axis of rotation of the linear hinge, but rather at a relatively small angle with respect to a radial transverse plane which is perpendicular to the said hinge axis.
In order to be able to increase the maximum freedom of rotation of the joint prosthesis with respect to the maximum freedom of rotation permitted by the extent of the cylindrical head itself (also known in the medical field as the flexural freedom), it is advantageous according to the invention, in the case where the cylindrical head is arranged on the end of at least one support arm which extends radially with respect to the cylindrical head, if one or both longitudinal edge(s) of the cylindrical socket are provided, in the area of the at least one support arm, with a cutout in which the cross section of the support arm can be completely or partially accommodated.
In order to prevent the functioning or effectiveness of the natural ligaments which hold the joint (in this case the joint prosthesis) together from decreasing, it is advantageous, according to the invention, if the freedom of rotation of the linear hinge in one or both direction(s) of rotation is greater than the associated freedom of rotation (medical: flexural freedom) of the natural joint which the joint prosthesis is intended to replace. This ensures that the freedom of rotation is limited not by the joint prosthesis but by the original, natural or possibly completely or partly artificial ligaments. The extent to which this freedom of rotation of the joint prosthesis is greater than that of the natural joint will preferably be approximately 5xc2x0 to approximately 20xc2x0, such as for example, and more preferably, approximately 15xc2x0.
In order in particular to design the joint prosthesis to be extremely small while nevertheless able to withstand high loads, it is advantageous, according to the invention, if the first part and the second part are each a metal part, for example are each a single-piece metal part. It is thus possible to produce joint prostheses which, in relative terms, are extremely small and yet strong, such as for example finger joint prostheses. The metal parts are in this case preferably made from a chromium-cobalt alloy, as is known per se from the prior art for prostheses and joint prostheses.
In order to improve the friction properties at the contact surfaces of the joint-prosthesis parts which can move with respect to one another, it is advantageous, according to the invention, if the first part and the second part, or at least the contact surfaces of the cylindrical head and cylindrical socket, are coated with a suitable coating material.
In order to counteract rejection and other undesirable interactions between the joint prosthesis and the surrounding human or animal tissue, it is advantageous, according to the invention, if the first and second parts, or at least their external surfaces, are made from or coated with a material which is compatible with the human, or if appropriate animal, body, i.e. in particular a material which is compatible with the human, or if appropriate animal, body without entering into chemical reactions therewith, or at least without entering into any adverse and/or undesirable chemical reactions therewith.
Since in many joints a so-called tilting or pivoting freedom is desirable or required only within a limited rotational position of the adjoining bones with respect to one another, it is advantageous, according to the invention, if the cylindrical socket extends over at least 160xc2x0, preferably over at least 170xc2x0, in the circumferential direction.
The joint prosthesis according to the invention can be used in particular as a finger joint prosthesis, such as an MCP prosthesis (MCP=metacarpophalangeal) or in particular a PIP prosthesis (PIP=proximal interphalangeal). Although prostheses are used relatively infrequently at the DIP joint (DIP=distal interphalangeal), since they are deemed less useful, it should be noted that the joint prosthesis according to the invention can also be used with considerable success as a DIP prosthesis. The same comments as those made in relation to the DIP prosthesis should also be noted in relation to toe joint prostheses, for which the joint prosthesis according to the invention can also be used with considerable success.
Depending on the specific location where the joint prosthesis according to the invention is used, it will be possible to adapt the orientation of the cylindrical socket with respect to the bone which is to be connected thereto, as well as the extent in the circumferential direction of the cylindrical socket, as well as the one or more cutouts for accommodating the support arm, in order to increase the freedom of rotation of the joint prosthesis in relation to the freedom of rotation permitted by the cylindrical socket.